1. Field of the Invention
Method of Pyrolyzing Sewage Sludge to Produce Activated Carbon.
2. Description of the Prior Art
The scarcity of acceptable land fill sites and recent incidents of beach pollution caused by ocean dumping of sludge have greatly increased the need to develop a different method for sewage sludge disposal.
One current method is to burn sewage sludge in multiple hearth and fluid bed incinerators, however, such incinerators require large quantities of auxiliary fuel and are environmentally unacceptable in some communities.
A primary purpose in devising the present invention is to supply a method of pyrolyzing wet sewage sludge that is substantially free of the operational disadvantages of prior art methods, and one that is environmentally acceptable. The present method, when carried out, produces activated carbon from the sludge that may be used to reduce solution in the water discharged from waste water treatment plants. If the sewage sludge prior to being pyrolyzed is dewatered sufficiently, a surplus of energy over and above that required to sustain the pyrolysis operation can be produced. The energy surplus is in the form of a combustible gas (pyrogas) that can be burned easily to generate steam or used to fire an internal combustion engine. Thus, the surplus energy can be used to supplement the electrical energy requirement of the treatment plant.
In the present method the sewage sludge, which is very viscous and sticky because of the high moisture content thereof, is mixed with recycled hot char resulting from practice of the method to produce a dry, free-flowing product, as well as the discharge of a substantial quantity of water vapor that is subsequently transformed to steam. The quantity of the char that must be recycled in the present method, depends on the initial sludge condition and operating condition of the apparatus used in carrying out the method. In the present method, only a first portion of the char in the form of activated carbon is recycled, with a second portion of the char in the form of activated carbon being removed from the apparatus from which the method is carried out, and this second portion being available for use in reducing the pollution in the water discharged from the waste water treatment plant. The present method has the operational advantage that heating of the ambient atmosphere adjacent the pyrolyzing operation is minimized by surplus heat from the operation being utilized to transform water to steam that is used to at least partially transform char from the operation to activated carbon. Also, a portion of the surplus heat may be used to preheat air that is mixed with the pyrogas prior to the latter being burned to sustain the pyrolysis reaction.